When water-thinnable resins, particularly aqueous basecoats, are pigmented, problems generally arise in that the pigments agglomerate or settle out. In order to produce stable water-thinnable lacquers, it is necessary that the pigments are satisfactorily wetted and are ground to a suitable particle size. The latter is generally less than 10 .mu.m. When water-thinnable dispersions with a low content of solvent are used, these aqueous dispersions can be destroyed by the action of shear forces. It is therefore necessary in many cases to replace part of the binder vehicles by special binder vehicles, which are termed paste resins and in which the pigments are ground, in order to effect wetting of the pigment or additives. These pigment pastes may be solvent-containing or water-containing substances. They are subsequently mixed with the remaining water-thinnable binder vehicles and thus form the aqueous coating medium, e.g. a primer, a primer surfacer or a base lacquer.
At the same time, it is necessary that aqueous coating media are stable on storage. Therefore they must not settle out, and there should be no formation of agglomerations of binder vehicles or pigments, which impair the uniform smooth appearance of the coating on the subsequent application thereof.
Water-tinnable coating media for metallic base lacquers or binder vehicles therefor which have an anionic basis are described in EP-A-0 260 447, EP-A-0 297 576, DE-A-40 00 889 and EP-A-0 438 090, for example. Proportions of the anionic base resin or neutral melamin resins. are used here as paste resins for the grinding of pigments, for example, or special paste resins are used which contain carboxyl groups incorporated by reaction. After neutralisation and conversion of the binder vehicles into the aqueous phase, the pigments are ground. Coating media can be produced with these aqueous pigment pastes.
DE-A-40 11 633, DE-A-26 06 831 and EP-A-0 251 772 describe aqueous coating media based on cationic resins which are at least partially neutralised. These can be used as an electro-dip lacquer or as an aqueous base lacquer. The pigments are ground in a non-ionic wetting agent together with organic solvents, for example. These pigment pastes are then mixed with a binder vehicle which contains cationic groups and are subsequently diluted to form an aqueous coating medium. In addition, cationic paste resins are also described, e.g. epoxy resins containing sulphonium groups which are employed for the production of pigment pastes, which are then processed with cationic resins to form a cationic lacquer material.
The coating media which have been described hitherto therefore only comprise those with cationic or anionic binder vehicles which contain additives or paste resins comprising the same ionic groups or comprising non-ionic groups.
DE-A-38 23 731 describes aqueous, thermally curable coating media which contain at least two binder vehicles containing oppositely charged ionic groups. With these coating media it is essential that the binder vehicles which contain differently charged ionic groups are immiscible with each other at storage temperatures, i.e. below the curing temperature. Two separate resin phases are therefore present. The binder vehicles only become miscible under curing conditions, i.e. at elevated temperature. The amounts of resins have to be selected so that the numbers of different ionic groups are present in about the same order of magnitude.
EP-A-0 459 634 describes binder vehicles which contain, simultaneously, substituents which can be converted into anionic groups and substituents which can be converted into cationic groups. The anionic groups are neutralised and the binder vehicle is converted into the aqueous phase. After the addition of pigments and neutralising agents, coating media are obtained. Mixtures of resins which are neutralised differently are therefore not described. CA-A-2 017 505 describes binder vehicles for the coating of paper, which are produced by the emulsion polymerisation of monomers containing cationic groups in an aqueous solution of a polymeric dispersing agent which contains neutralised anionic groups. The dispersing agent has to be neutralised in excess with a volatile amine in order to obtain a stable emulsion. The dispersing agent is present in amounts such that salt formation occurs on heating after the paper has been coated, whereby a dry film is formed.
For water-thinnable binder vehicles which contain ionic groups and which are used in the lacquer industry, it is necessary to develop a pigment paste, the polarity of which is matched to that of the binder vehicles in each case. This results in a considerable amount of costly development work.